1. Field of the Invention
The present invention relates generally to memory circuits, and more particularly, to a memory circuit for efficiently reading out font information stored in a read-only memory (referred to as ROM hereinafter) and transferring the same to a printer.
2. Description of the Background Art
FIG. 4 is a block diagram for explaining a structure of a printer comprising this type of memory circuit. A CPU (Central Processing Unit) 21 reads out a font required for printing processing based on a control program stored in a ROM 22 and controls driving of a printing portion or the like. A RAM (Random Access Memory) 23 stores data to be printed and work information stored in the CPU 21. A control bus 20, an address bus 24, and a data bus 25 respectively transfer a control signal, an address signal and a data signal, to exchange various signals among the circuits. A read-only memory (font ROM) 26 serving as a font memory stores a font pattern corresponding to a character code. A printing output circuit 27 receives font data read out from the font ROM 26 under the control of the CPU 21 and drives a printer head 28, thereby to print character information on a predetermined output paper.
FIG. 5 is a block diagram showing one example of a read-out circuit of the font ROM 26 shown in FIG. 4. A ROM array 31, which comprises, for example, a total of 65536 words.times.8 bits in 1024 rows and 64 columns, stores a font pattern to be a character corresponding to a character code.
A row decoder 32 decodes a row address signal transmitted through address lines A.sub.15 to A.sub.6 included in the address bus 24. The column decoder 33 decodes a column address signal transmitted through address lines A.sub.5 to A.sub.0 included in the address bus 24, to output the decoded column address signal to a column gate circuit 34.
A chip enable signal CE and an output enable signal OE are inputted to a timing generator 35 from the CPU 21 through the control bus 20. A timing output signal To which is an AND output of the signals CE and OE, to be inputted to an output buffer 36. Potentials on bit lines in a selected column is amplified in response to the signal To. Consequently, printing font data is outputted to 8-bit data bus lines D.sub.0 to D.sub.7 included in the data bus 25.
Referring now to a timing chart of FIG. 6, description is made of an operation of the printer shown in FIGS. 4 and 5.
The CPU 21 stores printing information in the RAM 23 based on a control program stored in the ROM 22. When a printing start command is inputted from the exterior, the CPU 21 outputs a row address signal and a column address signal to the row decoder 32 and the column decoder 33 through the address line A.sub.15 to A.sub.6 and A.sub.5 to A.sub.0 in order to extract to the data bus 25 font data stored in the ROM array 31 in the font ROM 26.
Consequently, an address to be accessed in the ROM array 31 is determined, so that the font data is read out to the column gate circuit 34. The font data is accepted in the output buffer 36 in response to a timing output signal To from the timing generator 35 (see the timing chart of FIG. 6). Then, printing font data is outputted through the 8-bit data bus lines D.sub.7 to D.sub.0 as parallel information, to be stored once in the RAM 23 through the data bus 25. Thereafter, transmission of the font data is synchronized with reception by the printing output circuit 27 and then, the printing font data is outputted to the printing output circuit 27 through the data bus 25 in response to a control signal of the CPU 21. The printer head 28 is driven based on the font data.
If and when the conventional memory circuit is utilized, particularly the ROM array is utilized as a font ROM, the above described memory access by the CPU 21 must be made several times until desired character font information is outputted, so that read-out efficiency is significantly decreased. Therefore, an attempt to deal with the decrease by increasing the number of output buffers 36 causes a disadvantage peculiar to the memory circuit that a circuit structure is complicated.
As described in the foregoing, the ROM array is most suitable as a program memory. However, the use of the ROM array as a font ROM is not efficient because reading processing is complicated.